1. Field of the Present Invention
The present invention relates generally to a toroidal type continuously variable transmission for e.g. an automotive vehicle. More particularly, the present invention relates to a technology for reducing surface pressure between a power roller and input and output conical disks of a toroidal type continuously variable transmission, and a technology for improving a transmitting efficiency between the power roller and the input and output conical disks.
2. Description of the Related Art
Previous toroidal type continuously variable transmissions generally have a constitution similar to that of a known toroidal type continuously variable transmission mentioned in "1989 JSAE Spring Convention Proceedings 1989-5", pages 167 to 170, published by Society of Automotive Engineers of Japan, in 1989.
FIG. 8 and FIG. 9 schematically show the configuration of the known toroidal type continuously variable transmission, wherein an input conical disk 1 and an output conical disk 2 are coaxially opposed to each other and arranged rotatably about a common axis O.sub.1, and at least one power roller (not shown) is arranged between mutually opposing conical faces 1a, 2a of the input and output conical disks 1, 2. The power roller is frictionally contacted with the conical faces 1a, 2a respectively at a contacting point. Such a contacting point is an oval surface in a strict sense, so that the contacting point is called a contacting oval. The power roller is arranged rotatably about a rotational axis extending across the above-mentioned axis O.sub.1, so that the power roller transmits rotational movement between the input and output conical disks 1, 2.
Further, as shown in FIGS. 8, 9, the power roller can be tilted about a tilting axis O.sub.2 extending perpendicular to the rotational axis of the power roller, in a bisecting plane M positioning perpendicular to the axis O.sub.1 between the conical disks 1, 2 (two examples of a tilting angle .PHI. of the power roller are shown by references .PHI..sub.1, .PHI..sub.2). By such tilting of the the power roller, each radius of tracing circles of the contacting point on the conical faces 1a, 2a can be continuously varied, so that a transmitting ratio between the input and output conical disks 1, 2 and hence a speed ratio between input and output speed of the toroidal type transmission can be continuously varied.
The known toroidal type continuously variable transmission is provided with a loading cam (not shown) for pinching the power roller between the input and output conical disks 1, 2 to enable the power roller to transmit the rotational movement. As shown in FIG. 8, the loading cam generates a thrust Fa corresponding to an input torque, which thrust Fa urges the input and output conical disks 1, 2 toward mutually closing directions, causing pressing forces exerted from the conical faces 1a, 2a to the power roller. The magnitude of the pressing forces for the power roller is varied as shown by references F.sub.1, F.sub.2 with regard to the conical face 1a, corresponding to change of degree of the power roller tilting angle e.g. from .PHI..sub.1 to .PHI..sub.2 even under constant magnitude of the thrust Fa, so that the pressing force for the power roller exerted from the conical face 1a is increased as the power roller tilting angle .PHI. is reduced. Thus, the pressing force for the power roller becomes large when the power roller tilting angle .PHI. enters into a small angle region causing lower speed transmitting ratios (higher value of the speed ratio between input and output speed of the toroidal type transmission to be available) corresponding to lower speeds such as e.g. 1st speed and 2nd speed of a conventional gear-type transmission, which small angle region is hereinafter called a lower speed transmitting ratio region. Incidentally, a large angle region of the power roller tilting angle .PHI. causing higher speed transmitting ratios corresponding to higher speeds such as e.g. 4th speed and 5th speed of a conventional gear-type transmission is hereinafter called a higher speed transmitting ratio region.
On the other hand, as shown in FIG. 9, in the known toroidal type continuously variable transmission, as the power roller tilting angle .PHI. is reduced e.g. from .PHI..sub.1 to .PHI..sub.2, a pressing direction distance from the contacting oval to the axis O.sub.1 is reduced e.g. from R.sub.1 to R.sub.2, resulting a surface area reduction of the contacting oval, while a pressing direction distance from the contacting oval to the tilting axis O.sub.2 is kept to a constant certain tilting radius R.sub.0. Because, the pressing direction distance determined along a straight line normal to the tilting axis O.sub.2 and passing through a center of the contacting oval and the tilting axis O.sub.2, is related to a curvature radius in a section of the input conical disk, which section being positioned in a hypothetical plane including the center of the contacting oval and the tilting axis O.sub.2 and extending across the rotational axis O.sub.1 of the input conical disk 1, so that the curvature radius in the above-mentioned section of the input conical disk and hence the surface area of the contacting oval is reduced corresponding to the reduction of the pressing direction distance.
Therefore, with a reduction of the power roller tilting angle .PHI. e.g. from .PHI..sub.1 to .PHI..sub.2, Hertzian surface pressure on the contacting oval is inevitably increased due to the increase of the pressing force and the surface area reduction of the contacting oval, thus the surface pressure on the input conical disk 1 and the power roller tilting angle .PHI. have a relation as shown in FIG. 10. Based on this relation, the surface pressure on the input conical disk 1 becomes remarkably high in the lower speed transmitting ratio region. Accordingly, in the known toroidal type continuously variable transmission, so as to reduce the surface pressure on the input conical disk 1 to allowable surface pressure for a sufficient durability, a torque capacity of the continuously variable transmission becomes inevitably small in the lower speed transmitting ratio region. Further, because reduction of the curvature radius of the conical disks 1, 2 and the power roller is restricted due to the small torque capacity in the lower speed transmitting ratio region, the surface pressure on the input disk 1 in the higher speed transmitting ratio region becomes small as shown in FIG. 10, so that a technique providing a reduced surface area of the contacting oval cannot be employed for an enhancement of a transmitting efficiency between the power roller and the input and output conical disks.
Another toroidal type continuously variable transmission is proposed by Japanese Patent Application Laid-Open No. 106456/88. In this known continuously variable transmission, as shown in FIG. 11, a main curvature radius defining the conical face 1a of the input conical disk 1 is enlarged to .beta. oversize (i.e. R.sub.0 +.beta.) in a region of the conical face 1a wherein the conical face 1a is contacted with the power roller in the higher speed transmitting ratio region, compared with a main curvature radius R.sub.0 in a residual region of the conical face 1a, while a main curvature radius defining the conical face 2a of the input conical disk 2 is enlarged to .gamma. oversize (i.e. R.sub.0 +.gamma.) in a region of the conical face 2a wherein the conical face 2a is contacted with the power roller in the lower speed transmitting ratio region, compared with a main curvature radius R.sub.0 in a residual region of the conical face 2a.
However, the known continuously variable transmission of the latter cannot solve the above-mentioned problem with reference to the surface pressure of the input conical disk in the lower speed transmitting ratio region that is severest for a durability of the input conical disk, because the known continuously variable transmission proposes neither technical means for reducing the pressing force F.sub.2 for the power roller shown in FIG. 8 nor technical means for increasing the distance R.sub.2 shown in FIG. 9. Further, the known continuously variable transmission of the latter cannot reduce surface pressure of the output conical disk 2 in the lower speed transmitting ratio region.